Fix #8487: Debugger confuses variables

To display the free variables for a single breakpoint, GHCi pulls out the
information from the fields `modBreaks_breakInfo` and `modBreaks_vars`
of the `ModBreaks` data structure. For a specific breakpoint this gives 2
lists of types 'Id` (`Var`) and `OccName`. They are used to create the Id's
for the free variables and must be kept in sync:
If we remove an element from the Names list, then we also must remove the
corresponding element from the OccNames list.

2 files changed, 6 insertions, 4 deletions

diff --git a/compiler/ghci/ByteCodeGen.hs b/compiler/ghci/ByteCodeGen.hs
index 8b23e08003..12331e2d52 100644
--- a/compiler/ghci/ByteCodeGen.hs
+++ b/compiler/ghci/ByteCodeGen.hs
@@ -432,8 +432,8 @@ schemeER_wrk d p rhs
         return $ breakInstr `consOL` code
    | otherwise = schemeE d 0 p rhs
 
-getVarOffSets :: DynFlags -> StackDepth -> BCEnv -> [Id] -> [(Id, Word16)]
-getVarOffSets dflags depth env = catMaybes . map getOffSet
+getVarOffSets :: DynFlags -> StackDepth -> BCEnv -> [Id] -> [Maybe (Id, Word16)]
+getVarOffSets dflags depth env = map getOffSet
   where
     getOffSet id = case lookupBCEnv_maybe id env of
         Nothing     -> Nothing
diff --git a/compiler/ghci/ByteCodeTypes.hs b/compiler/ghci/ByteCodeTypes.hs
index 628b576ca0..0c0c34ad64 100644
--- a/compiler/ghci/ByteCodeTypes.hs
+++ b/compiler/ghci/ByteCodeTypes.hs
@@ -35,6 +35,7 @@ import Data.Array.Base  ( UArray(..) )
 import Data.ByteString (ByteString)
 import Data.IntMap (IntMap)
 import qualified Data.IntMap as IntMap
+import Data.Maybe (catMaybes)
 import GHC.Exts.Heap
 import GHC.Stack.CCS
 
@@ -110,14 +111,15 @@ instance NFData BCONPtr where
 -- | Information about a breakpoint that we know at code-generation time
 data CgBreakInfo
    = CgBreakInfo
-   { cgb_vars   :: [(Id,Word16)]
+   { cgb_vars   :: [Maybe (Id,Word16)]
    , cgb_resty  :: Type
    }
+-- See Note [Syncing breakpoint info] in compiler/main/InteractiveEval.hs
 
 -- Not a real NFData instance because we can't rnf Id or Type
 seqCgBreakInfo :: CgBreakInfo -> ()
 seqCgBreakInfo CgBreakInfo{..} =
-  rnf (map snd cgb_vars) `seq`
+  rnf (map snd (catMaybes (cgb_vars))) `seq`
   seqType cgb_resty
 
 instance Outputable UnlinkedBCO where